Sparking plug



Feb. 24, 1948. P. E. HALSTEAD SPARKING APLUG Filed Apri; 28, 1944 e W E EEE Patented Feb. 24, 1948 SPARKING PLUG:l

Philip Edward Halstead, Hammersmith, London, England, assignor to West Road Co. Limited Application April 28, 1944, Serial No.

In Great Britain June 11, 1943 1 Claim.

This invention relates to sparking plugs of the kind which incorporate a vresistance element as part of the electrical circuit for the purpose of inhibiting electrode erosion and/or suppressing interference with wireless receiving apparatus,

and the invention has for its object to provide l improvements in the known type of sparking plug wherein an electrode housed within the insulator comprises upper and lower portions having interposed therebetween a resistance element that is in electrical communication with the said upper and lower portions.

In my improved sparking plug the insulator comprises ceramic material and the resistance element comprises ceramic material which is semiconductive or which is rendered semi-conductive, the resistance element being bonded-to the insulator to form an integral part thereof by being red in situ in the insulator.

The insulator is formed of ceramic material of high electrical resistance and is preferably of the sintered oxide type. It may consist predominantly Aof aluminium oxide. The resistance element may be formed of ceramic material which is, per se, semi-conductive, or of ceramic material, e. g. olas/, which is of high resistivity and which has been rendered semi-conductive by incorporating therewith carbon, metallic carbides or other suitable conducting materia1 or by providing a rod or other formed body of the insulating material with a conducting iilm or coating, e. g. of platinum or carbon.

The resistance element preferably has a resistance at room temperature of not less than 500 ohms, and the resistance may advantageously be between 1000 and 2000 ohms, say 1500 ohms,

Where the inhibition of electrode erosion is the main consideration. For the suppression of in- 'terference with radio apparatus, it is preferred that the resistance element should have a considerably higher resistance, say 15,000 to 50,000 ohms.

It is preferred that the resistance element and the insulator should have substantially the same coeiilcients of thermal expansion, for example, the insulator and resistance element may have essentially the same composition. In this manner a, very satisfactory seal for the central electrode may be obtained having little or no tendency to fail during use of the plug.

My invention will be further described with reference to the accompanying drawing, in which Fig. 1 is a central vertical sectional view of one form of insulator assembly in accordance with my invention, and

(Cl. 12B-169) Fig. 2 is a similar view of a modified form of insulator assembly.

Referring iirst to Fig. 1, the insulator i shown therein has a centrally disposed bore 2 and a counter bore 3. The lower portion 4 of the central electrode is housed within bore 2 and has at its upper end a flange which seats on the ledge formed at the junction of bore 2 and counterbore 3. Housed within the lower part of counterbore 3 is the resistance element 5, which is bonded to the insulator i, whilst the upper portion 6 of the central electrode isthreaded into the counterbore. The resistance element 5 is in electrical communication with the two portions 4, E of the central electrode and in the form of the insulator assembly shown in the drawing a thin layer 1 of silver, graphite or other material of high conductivity is interposed between the resistance element and the adjacent ends of the central electrode sections in order to provide good electrical contact between the resistance element and the electrode sections.

In making the insulator assembly shown in Fig. 1, a finely ground ceramic body mixture having,

`for example, the composition:

Per cent A1203 92 S102 n 7 MgO 1 is formed to the shape of the insulator shown in the drawing. After inserting the lower portion 4 of the electrode in the bore 2 of the green body, a small quantity of powdered silver, graphite or other suitable material is placed Within the counterbore. Thereafter there is placed within the counterbore the material which is to form the resistance element 5 and which may have the composition:

Per cent Clay Silicon carbide 20 Graphite 10 casacca sage of gases and vapours. In a modied method of making the insulator assembly, the insulator and resistance element are formed to shape and red separately. The components of the insulator assembly are then assembled and the assembly is lred to bond the resistance element to the insulator.

In the modified form of insulator assembly il lustrated in Fig. 2, the resistance element comprises a rod 5 of ceramic material, preferably of the same composition as that of the insulator i, which rod is rendered semi-conductive by a thin peripheral lm or .layer 3 of platinum, carbon or other conductor o! electricity, and the resistance element is bonded to the insulator i by a layer of glaze 9 applied to the side of the resistance ,element In other respects this assembly is similar to that shown in Fis. 1.

In making the insulator assembly shown in Fig. 2, the insulator i and rod E' may be formed and iired separately. The rodv 5' may then be coated all over with a suitable compound which, on heat= ing, will decompose to leave on the' rod a thin film of the desired conductor. The-rod 5 is then heated to provide the conducting' nlm after which it is coated with a suitable glaze composition. After assembling the coated roti and the electrode sections in the insulator, preferably with. a layer of graphite, silver powder or the like interposed between the rod and the electrode sec tions, the assembly is red to cause the glaze to bond the resistance element to the insulator.

I elaim:

A smrlrlng plug of the kind specied comprising an insulator formed of ceramic material, an electrode housed within the insulator and having upper and lower portions, and a resistance element interposed between said upper and lower portions and in electrical communication therewith, said resistance element comprising a body portion of ceramic material having substantially the same coemcient oi thermal expansion as the insulator. saldbody portion having a coating of conducting material to render the element semiconductive, and a layer of glaze surrounding said conducting coating, said resistance element being re bonded to the insulator through said glaze.

'I' EDWARD HALSI'EAD.

RMENCES mit@ The following references are of record in the 

